We are investigating the role of cyclic nucleotides in the retina and their relationship to vision. We have found that rod outer segments, the retinal photoreceptor units, are unique in containing a 100-fold higher concentration of cyclic GMP than cyclic AMP, a ratio that is reversed in all other tissues of the body yet studied. Outer segments also contain a specific GTP-dependent kinase activity which appears to perferentially phosphorylate rhodopsin. Its activity is increased almost 50-fold by light exposure. Guanine nucleotides may thus be important in the visual process and/or in other normal photoreceptor functioning.